1. Field of the Invention
The present invention relates to a printing media feeding apparatus, a printing apparatus provided with the feeding apparatus, a printing media feeding speed control method, and a computer program.
2. Description of the Related Art
In related art a thermal printer with a thermal head has a thermal head in which a plurality of heat elements are linearly arranged and controls power distribution to the heat elements depending on the tone level to heat a heat-sensitive recording layer, thereby printing an image on printing media.
In such a thermal printer, color density depends on the energy applied to the printing media. That is, to obtain deep color, the heating value of the heat elements is increased; to obtain light color, the heating value thereof is reduced.
The energy to be applied to the printing media is increased when the feeding speed of the printing media is low. In this case, the obtained color may become deeper than a desired level. On the other hand, the energy to be applied to the printing media is reduced when the feeding speed of the printing media is high, with the result that the obtained color may become lighter than a desired level.
As shown in FIG. 1, speed irregularity of the printing media appears as density irregularity in an image printed on the printing media, the density irregularity being formed by a plurality of lines running in the direction perpendicular to the feeding direction of the printing media. In FIG. 1, an area 101 is a low-density part, and area 102 is a high-density part. It is therefore demanded in the thermal printer that the speed irregularity in the feeding operation of the printing media be eliminated.
In general, the printing media feeding apparatus uses a stepping motor as a drive source to rotate a capstan of the final stage through a drive transmission mechanism such as a pulley, an endless belt, and a gear. The capstan feeds the printing media while holding the printing media together with a roller provided opposite thereto. Since a means for directly feeding the printing media is the capstan, the rotational speed of the capstan needs to be constant in order to feed the printing media at a constant speed.
However, it is difficult to make the rotational speed of the capstan constant, since it depends on the mechanical accuracy of the drive transmission mechanism. For example, the rotational speed of the capstan comes under the influence of the rotational accuracy of the pulley, the rotational accuracy of the capstan itself, and the like. However, even if the drive transmission mechanism has been mechanically assembled with high accuracy, it is still difficult to make the feeding speed of the printing media constant and eliminate the density irregularity.
As prior art documents related to a technique of eliminating the density irregularity, the following patent documents are known: Jpn. Pat. Appln. Laid-Open Publication Nos. H11-334160, H5-169708, 2001-239686, and S63-296976.